Electron tube grids and method of making the same



Jan. 5, 1965 J. J. MoscoNY ETAL 3,164,740

ELEcTRoN TUBE caws AND METHoD oF MAKING THE: SAME Filed April 29. 1960 United States Patent O 3,354,749 ELECTRN TUBE GMES AND METHOD F MAKER; THE SAME John loseph Moscony, Lancaster, and Robert William Etter, Lititz, Pa., assignors to Radio Corporation of America, a corporation of Delaware Filed Apr. 29, i969, Ser. No. 25,682 11 Claims. (Cl. 313-348) The present invention relates to electron tube grids and particularly to grids useful in tubes suited for relatively high power applications and methods for making the same. Specicially, the invention concerns a grid structure having an improved coating thereon for inhibiting both primary and secondary emission therefrom.

Due to the high temperature environment in which grids operate in electron tubes of relatively high power, such grids are usually made of a refractory metal having high hot strength, such as molybdenum or alloys thereof, and are coated with a metal having a relatively high Work function, such as platinum, to reduce primary and secondary emission therefrom.

One of the problems encountered in grids comprising a base of refractory metal such as molybdenum having a coating thereon of a high work function metal, such as platinum or gold, is that the high work function metal diffuses appreciably into the base metal either during operation of the tube or during its manufacture. Such diffusion produces an alloy which has an appreciably lower hot strength than the molybdenum base. Consequently, an appreciable diffusion of the high Work function metal into the refractory metal base, weakens the grid structure during tube operation to such degree as to result in deformation of the grid, thereby rendering the grid incapable of performing its intended function.

In addition to weakening the grid structure, the diffusion aforementioned may proceed to the point where the outer surface of the grid structure constitutes the diffusion alloy referred to, thereby defeating the attainment of a high work function grid surface.

The foregoing problem of diffusion of the coating into the base is particularly severe when the grid base constitutes a structure drawn from molybdenum sheet metal. ln this situation, the crystals in the molybdenum base are appreciably elongated and indeed, the surface crystals engaged by the drawing tool, have been found for the most part to have ruptured, leaving a small space between the resultant crystal fragments. The application to such surface of a coating of high work function metal, such as platinum, results in migration of an appreciable portion of the coating material into such small spaces between fragmented crystals and into engagement with the adjacent ragged end surfaces of the crystal fragments thereby reducing the thickness of the coating on surface regions between the spaces referred to. Such reduced thickness and the larger area of coating engagement with the base presented by the end surfaces referred to, increases the likelihood of complete coating diffusion into the base. Such complete coating diffusionV destroys the high work function surface sought to be provided by the coating.

As a consequence of the foregoing problem, it has heretofore been considered desirable to utilize relatively thick coatings of high work function material. Such thick coatings, however, have resulted in objectionable incidents of coating peeling, and have involved appreciable expense due to the high cost of the coating material.

Accordingly, it is an object of the invention to provide an improved grid for electron tubes operable at relatively high power.

lt is a further purpose to provide a grid having relatively high hot strength and a surface of high work function.

Another aim is to provide a grid comprising a molybdenum base having thereon a material of high work function that is characterized by substantial freedom from diffusion into the base.

A further object is to provide an improved method of making a grid from which primary and secondary emission are substantially inhibited during high temperature operation.

One example of a grid according to the invention comprises a structure including a base of molybdenum having thereon a first relatively thin coating of rhodium sintered to the base, and an outer coating of platinum. Applicants have found that rhodium exhibits substantial freedom from diffusion into the molybdenum base and possesses the desirable characteristic of reconstituting the fractured surface crystals into units having appreciably higher strength than the crystal fragments had before reconstitution. This contributes not only to a higher hot strength of the grid, but also results in a lirm anchoring of the rhodium coating to the base.

The rhodium coating is applied to a thickness suflicient to reconstitute the crystal fragments aforementioned, and to coat the surface of the base. Since rhodium is substantially free from diffusion into the molybdenum base, whether made of complete crystals or crystal fragments, relatively thin coating is feasible for fully covering the ase. stantially free from diffusion into platinum. Consequently the relatively thin coating of rhodium is substantially free from alloying with an outer platinum coating. As a result, the thin rhodium coating serves as an effective barrier to platinum penetration and diffusion into the molybdenum base. Futherrnore, the substantial freedom of the rhodium coating from diffusion into either the molybdenum base or the platinum outer coating, preserves the anchored bond of the rhodium to the molybdenum. In addition, the function of the rhodium of uniting fractured surface crystals of the molybdenum is unimpaired.

The recognition by applicants that the foregoing characteristics of rhodium are of appreciable advantage in the art of grids for electron tubes of relatively high power output, is a significant contribution to this art. Experimental tubes in which a grid according to the invention has been incorporated have demonstrated a remarkable stability of the grid in respect of freedom from coating peeling and from primary and secondary electron emission at relatively high temperatures of tube operation, and the high hot strength of the grid has been evidenced by good tube operation during long life tests of the tubes under operating conditions.

Further features and objects of the invention will become apparent as the present description continues.

ln the drawing, to which reference is now made for a more detailed consideration of the invention',

FIG. l shows an elevation in cross-section of a multiple grid assembly wherein the grids are in the form of drawn cups of molybdenum sheet metal coated in accordance with the invention;

F-lG. 2 is a sectional view taken yalong the line 2 2 of FlG. 1, and shows radially registering openings in the two grids constituting the `assembly shown in FIG. 1;

FIG. 3 -is a greatly enlarged schematic representation of a fragment :of one of the grids shown in FIG. 1, and depicts a surface and sub-surface condition of -a grid according to the invention; and

FIG. 4 is a graph showing the improvement in suppression of grid emission realized by a grid incorporating the invention.

FIG. 1 shows a grid assembly which may be employed in a power tube of the type shown in copending application SN. 769,646, filed October 27, 1958, now Patent 2,951,172, and assigned to the assignee of the instant ap- Applicants have found that rhodium also is sub- 'I d plication. The tube Ithere shown by way of example is a tetrode type,- including an external air cooled anode, a directly heated vwoven cylindrical cathode, and a grid assembly.

The grid assembly, :as shown in FIG. 1, includes a control grid l@ having an activeportion 11 and a conical support portion l2 terminating in a flange 114. The flange 14 is sealed to the lower face, as viewed in iFlG. 1, of a ceramic spacing ring i6. cludes an active portion 2t?, a conical support portion 22 and a sealing iiange 24 sealed to the upper surface of ceramic ring 16. The assembly also-includes a ceramic cylinder 26 sealed to `screen grid'ilange 24, and to a metal iange 23 to which the anode (not shown) may be fixed as fby welding or brazing.

The active portions l1, of the grid structures shown in FIG. 1 are made of molybdenum. The conical portions 12, 22 a-nd the sealing flanges 14, 24 are made of copper for improved heat conductivity from the active grid portions aforementioned. The active grid portions 11, 29 are sealed to their associated conical portions l2, 22, `-by means of brazes 29, 31 made of an alloy comprising 371/2 percent gold and 621/2 percent copper. 'This alloy has a melting point of about 950 C. The cup-shaped vactive grid portions 1l, '29 may be drawn with suitable dies from la fbody of molybdenum in `sheet metal form.

After the active :grid portions have been drawn to desired cup-shape, they `are brazed to the conical portions serving as supports as by the copper-gold brazing material aforementioned. The sealing flanges are suitably sealed as bylbrazing to the ceramic spacer members '16, 26, while supported in concentric relation by means of a suitable jig (not shown). The brazing material for the latter seals .may be of a type known commercially as BTV Braze, comprising lan alloy of 28 percent copper and 72 percent silver and having la melting .point of 779 C. After the parts have been b-razed and while lthe parts are supported in the aforementioned concentric relation, radially registering apertures 20, sho-wn in FIG. 2, are cut through the active grid portions 11, 2t?, by lsuitable means,

such `as electrical discharge machining.

During the drawing of the molybdenum sheet metal to cup shape, crystals of molybdenum 30 (FIG. 3) are appreciably elongated at surface and sub-surface portions of the molybdenum sheet metal body 3f. Such elonga tion is a consequence of t le mechanical reaction between the molybdenum sheet metal body and the dies employed in drawing the body .to cup shape. Such reac-tion is most severe at the surface of the molybdenum engaged by a die, and has been found by applicants to result in rupture of the elongated surface crystals.V Such rupture of a crystal produces two crystal fragments 32, 34 separated by a space 36, and results in appreciable weakening of the structural strengthof a vgrid in which such surface crystal rupture has occurred. When the material of sucn ygrid is `relatively thin and the grid is operated at relatively high temperatures, such crystal rupture may result in grid translations adversely laffecting the operation of a tube in which the grid is used.

It is apparent from 3 that if a metal coatingris applied to the surface of molybdenum base 31, its `area of Contact with molybdenum is appreciably increased lby the fragmentation of the surface crystals in the base. Such area not only includes side surfaces of the elongated crystals 32., 34, but also ragged end surfaces 36, 38, produced `on rupture of a crystal. Such increased area of contact of the coating with the ibase, increases the area of likely diffusion of the coating material into the materialiof the base. Where the coating material constitutes platinum, such'difusion is appreciable and results in a relatively thick layer of an alloy of molybdenum and platinum. than molybdenum, but the alloyingacticn may proceed through the coating and thereby yadversely affect the inhibition of primary yand secondary emission. A resort to A 4screen grid 13 in-V Such alloy not only is mechanically weaker a relatively thick platinum coating does not satisfactorily solve the latter problem, since it provides a larger reservoir of platinum for diffusion into the molybdenum there- Iby increasing the thickness of the alloy layer and further seriously impairing the mechanical strength of the grid. Furthermore, the thicker coating introduces problems of peeling.

Applicants have found that when rhodium is applied 't0 a drawn molybdenum Igrid to a thickness of from 5 X10-7 inch to 5 16 inch, and heat treated according to the invention, it forms :a coating d@ which not only fills the spaces 36 between molybdenum crystal fragments, but also covers the entire outer surface of the base defined by the exposed sides of the'fragmen'ted elongated surface crystals 32, 34. The thickness value referred to, characterizes the portion of the coating covering the exposed sides aforementioned. The portions of the coating at the regions of the spaces 36, is slightly thicker.

While the thickness .dimension aforementioned of the rhodium coating is extremely small, applicants have found the outer surface of the coating to be free of any traces of molybdenum, .thus indicating with respect to diffusion of the rhodium into the molybdenum base, that its magnitude has been )sufficiently small so .as to preserve the purity of the outer surface of the coating.

Applicants have further found that when rhodium is applied to a thickness within the range specified, to a drawn molybdenum base by the method to be described, the rhodium reacts molecuiarly with the crystal fragments 32, 34 so as to reconstitute the fragments into a new crystal. Such new crystal is therefore composed of the fragments 32, 34 and a body 42 of rhodium. Such reconstitution of the crystal fragments mechanically strengthens the molybdenum base, and at the same time, results in a strong anchored engagement of the rhodium coating ttl to the molybdenum base 31.

in practicing the method of the invention it is important to note that the coating application should take place after all mechanical working of a'grid work piece has been completed. In the instant example, therefore, it is necessary that'the apertures 28 in the grid be provided prior to the coating application. Otherwise, edges exposed by the aperture-forming step'wouid be free of coating material and constitute a source of emission. But since the apertures are not produced until after the two-grid assembly shown in FIG. 1 is completed by fixing the two grids with respect to each other, the coating operation in vthis example will be performed on the two-grid asseL bly as a work piece.

However, only a portion of the work piece shown in PEG. 1 requires a coating of emission inhibiting material according to the invention. This portion comprises the upper portion of the work piece, as viewed in FIG. 1, defined by the active grid portions 11 and Zd.

This upper portion is initially prepared for the plating step by any suitable electropolishing and/ or etching media. rfhereafter the upper portion referred to is disposed downwardly in a plating bath consisting of one to two grams of rhodium as metal in an aqueous solution containing ten to thirty grams of concentrated sulfuric acid per liter, and maintained ata temperature of from 40 C. to 60 C. The bath is then subjected to a current density of from 10 to 30 amperes per square foot, for a period of from one to ten minutes. v

The Work piece, coated with rhodium in accordance with the foregoing is then heated at a temperature of from 500 C. toV 750 C. in a reducing atmosphere, such as dry hydrogen, for a period of from thirty minutes to one hour, for heating the rhodium coating. It is believed that the heating step is important in effecting reconstitution of fragmented crystals in the surface of the molybdenum base, and for rmly anchoring the coating to the base. Of Vparticular significance is the reducing atmosphere in ywhich the heatingVV step is carried out. lt is believed that this atmosphere, by reducing residual oxides on crystal edges exposed on fragmentation of the surface crystals of the base, contributes appreciably to the reconstitution of the ruptured crystals.

The reason Why the temperature of 750 C. is chosen as the maximum is to avoid impairment of previously made seals between the ceramic spacers and copper flange of the grid. While it is possible that certain active portions of the grid attain a temperature of a thousand degrees or more in tube operation, such temperatures are not obgectionable to the functioning of the rhodium coating described before herein. This is so because the thickness of the rhodium coating is suliicient to provide an adequate reserve of unalloyed rhodium which is available to permit further diffusion of the base and outer coating materials theteinto without fully penetrating the rhodium coating during the useful life of the tube.

Stich active portions which attain the temperatures aforementioned are remote from ends of the grids which are brazed to the conical support. The heat conducted to the ends of the grids that are brazed to the conical support is dissipated partly by conduction through the conical support, which is made of a relatively high heat conducting material such as copper. In this way the temperature at the grid ends referred to, is at a much lower temperature than that of the aforementioned certain portions of the grid.

After the heating step, and with the work piece returned to room temperature, a coating of platinum s3 (FlG. 3) is electroplated over the rhodium coating. One platinum plating bath, for example, may consist of an aqueous solution of chloroplatinic acid consisting of tive to twenty grams per liter of chloroplatinic acid salt. The plating bath also includes from l0 to 20 grams per liter of boric acid, and from 30 to 50 grams per liter of ammonium phosphate. When this platinum bath is used, the temperature of the bath should be from 70 C. to 90 C. and the current density from to 30 amperes per square foot. The portion of the work piece previously plated with rhodium should be allowed to remain in the bath at a current density within the range specified, for a length of time sudicient to provide a platinum coating having a thickness of from 0.00005 to 0.0001 inch. Such length of time is about l0 minutes.

Another plating bath that may be employed in the platinum coating step may consist of an aqueous solution containing 6.5 grams per liter of patinum diamino` nitrite and l0() grams per liter of disodiurn phosphate and grams per liter of diammonium phosphate. This bath should be kept at a temperature of from 80 C. to 95 C. during the plating step. The current density used should be from 50 to 100 amperes per square foot. The worlc piece should be permitted to remain in the bath at the current density indicated for a suilciently long time to produce the platinum coating thickness aforementioned. This time has been found to be about minutes.

The platinum coating applied in accordance with .the foregoing is free from mechanical stress and shows no tendency to blister or peel from the intermediate rhodium Coating.

The remarkable degree to which a grid made in accordance with the invention is free from emission is shown graphically in FIG. 4. The two curves 44, show, from tests, the emission characteristics of the screen grid in two tetrode type tubes operating at substantially the same temperature and anode potential and having substantially the same structure, except that in one (curve ed), the screen grid has a coating of carbon applied in a conventional way, and in the other (curve 46), the screen grid is coated in accordance with our invention. Both screen grids have molybdenum bases and comprise sheet metal structures drawn to cup shape and having apertures, as shown in FIGS. 1 and 2. The screen grid is selected for purposes of comparison since alegran this grid tends to be more sensitive in respect of emission than other grids, such as the control grid.

As shown by curve 46 in the graph of FIG. 4, a screen grid in accordance with the invention is characterized by a relatively small magnitude of emission, i.e. from about 25 to about 50 microamperes throughout a range of power input of from 50 to 300 watts. A conventional screen grid having an outer coating of carbon, `on the other hand, exhibited a sharply rising curve of grid emission of from about 25 microamperes at a power input of about 50 watts to about 600 microamperes at a power input of about 300 watts, as shown by curve 44.

ln tubes of the type including control and screen grids having apertures in accurate radial register, the problem of primary emission from the screen grid is more serious `than that of secondary emission. This is because the effective areas of the screen grid are shielded by the control grid from electron impingment, thereby eliminating, or substantially reducing incidents of secondary emission. The curves shown in FIG. 4 are therefore primarily representative of primary emission from the screen grid.

Such primary emission is dependent on the temperature of the screen grid and the work function of its surface. in relatively high power tubes of the type discussed, the anode is characterized by a relatively high temperature in operation. ince the screen grid is relatively close to the anode, it acquires heat by radiation from the anode and assumes a temperature close to that of the anode.

This heat, at the various power inputs shown in FIG. 4 is sutlicient to cause a screen grid having a surface of notoriously high work function material such as carbon, to be characterized by a relatively high order of primary emission. in the same environment, however, a screen grid having the improved coating of the invention, exhibited very little primary emission.

This difference in emission, depicted in FlG. 4, is therefore due solely to the surface condition of the screen grid. This surface condition according to the invention is determined to a great extent by the intermediate coat of rhodium, and particularly by the thinness of such coating within the range specified before herein. Such thinness prevents the build-up of strains in the coating that seek relief in cracks causing coating peeling. Applicants have found that such thinness is feasible due to a combination of factors such as the removal of oxides from the molybdenum base during the heating operation, as explained herein, and applicants discovery that a rhodium coating of the thinness indicated is free from :that degree of diffusion into the base material that would affect adversely the purity of the outer surface of the coating.

Applicants have found that rhodium as an intermediate coating, also opposes diffusion into outer coating metals exhibiting desirably high work function, such as platinum, gold, palladium and other noble metals. This is desirable in that the rhodium coating may be characterized by the thinness referred to, for good adherence to the base, while .preventing diffusion into the base of such high work function metals. The outer coating of high work function metal is therefore characterized by an exposed surface of high purity for improved inhibition of emission.

While the initially applied rhodium coating presents a surface of high work function and might be considered suitable per se to provide a grid in which emission is inhibited, it is preferred not to rely solely on the initial rhodium coating for this purpose. This is because the thinness of such coating found desirable in practicing the invention, is such as to present hazards of coating erosion by even the slight degree of electron impingement to which the screen grid may be subject. Consequently, it is preferred to add a second coating of high worlr function material to the grid. However, this second coating may be relatively thin, as before indicated.

The second coating may be made of any high Work function material, such as the metals described before ond coating consistsV of platinumhaving herein. Rhodium may also be used as the material of thinness within the range indicated, wherein the diffusion does not extend through or involve the entire thickness of the rhodium coating. Consequently, the rhodiumcoating according to the invention presents a purersurface region to the base and another pure surface region to the outer coating. It is fortuitous that rhodium is characterized by the aforementioned limitation on its diffusive reactance at a thinness Vat which it exhibits good adherence to the base.

The references in the foregoing and in the appended' claims, to molybdenum as the material of the grid base to which coatings are applied, are meant to comprehend molybdenum and alloys containing a substantial proportion of molybdenum.

While the foregoing example has involved the utilization of molybdenum or a molybdenum alloy as the base material for a grid, it is also feasible to practice the invention in association with tungsten as the base material.

Where tungsten is used as a base material for a grid, it is not practical to shape the base by drawing, as in the case of molybdenum, because of the greater hardness of tungsten. One way, however, in which tungsten can be formed to desired shape is by rolling. One shape that can be formed by such operation is that of a rod-like structure (not shown). A plurality of Vsuch structures when ,suitably supported may serve'as a grid in a tube of the super-power type, for example.

A rolled tungsten structure is characterized by a crystal deformation in the surface thereof somewhat similar to that produced in drawn molybdenum aforementioned. lt has been found that rhodium, when applied as a coating in accordance with the invention to rolled tungsten, serves to reconstitute fragmentedsurface crystals Vand thereby to strengthen the structure. Rhodium may be applied to a base of rolled tungsten, prepared as in the case of molybdenum, by using plating baths and current densities and a heating step similar to those employed for coating molybdenum. However, since the rolling of tungsten produces compressive strains therein, the thickness of they coating and therefore duration of coating application are not as critical as in the case of molybdenum. Thus rhodium may he applied to sufiicient thickness so as to constitute the solecoating on the tungsten base'. However, in the interests of economy, it is preferred to apply the rhodium coating to the thickness specified in the foregoing for a molybdenum base and to apply a further coating of gold.

It is believed that rhodium serves as a catalyst during the aforementioned heat treatment, in reducing oxides remaining on the base material after the initial Vpreparation thereof, thereby promoting a more complete reduction of such oxides than is possible with other high work function materials. Such more completereduction of the oxides not only facilitates the reconstitution of ruptured surface crystals in the base material, but also contributes to the improved adherence of the coating to the base.

We claim: l. A grid for an electrontube comprising a refractory metal structure having a first coating thereon of rhodium having a thickness of from 5 l0'I to 5 l06 inch and a second coating of a material having a relatively highl work function on said'first` coating.

2. A grid according to claim l, and wherein said, seca thickness of `from 5 X10-5 to 1x10-4 inch. q

3. An electron tube grid comprising a refractory metal structure'having an outer coating of a high Work function metal, said structure having an intermediate coating of rhodium, said coating of rhodium being nearly completely ditised into said structure and said outer coating and sufficiently thick for preventing the formation of an alloy including both material of said structure and material of said outer coating. y

4. A grid for a power type electron tube comprising a base made of molybdenum, a first coating of rhodium on said base, and a second coating of la material having a relatively high work function,` said second coating having a thickness of' from 5x10-5 to 1x10-i inch, said first coating having Va thickness of from about 1/2 percent to 5 percent of the thickness of said second coating.

5.l A grid for a power tube comprising a drawn molybdenum structure in which the surface thereof constitutes spaced crystal fragments, said structure having thereon a first coating of rhodium, said rhodium coating having a thickness filling the spaces between said frag ments for reconstituting said fragments into completecrystals and covering exposed sides of said fragments to a thickness for optimum adherence of said coating on said structure and atwhich said coating is free from diffused penetration therethrough by the material of said structure, said structure having a second coating on lsaid first coating, said second coating being made of a material having a relatively, high Work function.

6. Method of making a grid characterized by reduced emission and high strength at temperatures up to l000 C., comprising the steps of applying on a molybdenum grid base, a coating of rhodium to a thickness of from 5 107 Vto 5 10-6 inch, heating said rhodium coating in a reducing atmosphere and at a temperature of from 500 C. vto 750 C. from about thirty minutes to about one I* hour, and thereafter applying to the heat treated coating a coating of a high work function material.

7. Method of making a grid of low emission when operated at relatively high temperatures, said method comprising the steps of drawing molybdenum sheet metal to cup shape, fixing said drawn cup in a grid mount, puncturing said drawn cup vto provide apertures therein, coating saidV apertured drawn cupV withV a relatively thin layer of rhodium, heating theV rhodium layer in a reducing atmosphere at a temperature of from 500 C. to 750 C. for a sufficiently Vlong time to cause Vsaid rhodium to reconstitute surface crystals of said grid cup fragmented dur-Y ing said drawing step and toV cause a portion only, of said rhodium layer having a Small thickness in relation to the thickness of said rhodium layer, to diffuse into the material of said drawn cup for improving the adherence Vof said layer to said cup, and thereafter coating the rhodium coated grid cup with a relatively thick layer of platinum, said layer of rhodium being sufiiciently thick to prevent diffusion therethrough to said portion of material from said platinumy layer.

8. Method of making-a coated molybdenum grid, comprising drawing a molybdenum Work piece to grid shape, electroplating said grid shape With a strike layerrof rhodium having a thickness of from 5 l0Fl to 5X 10-s inch, heating said layer in a reducingatmosphere at a temperature ofl from 500 C. to 750 VC. for about thirty minutes to about one hour, and electroplating on said Vheat treated layer another layer of platinum to a thickness or from 5 l0h5 to 1 l0t inch.

' 9. In a method of making an assembly including two concentric grids, comprising drawing two cylindrical cups of dliferent diameter from a body of molybdenum sheet metal, fixing said cups in concentric relation, providing layer of rhodium, heat treating said layer of rhodium in a reducingatmosphere, and coating said heated layer with a coating of relatively high Work function material to a thickness greater than the thickness of said heated layer.

10. In a method of coating a molybdenum electrode work piece with a relatively thick layer of platinum, the steps of coating said Work piece with a relatively thin intermediate layer of rhodium and heat treating the said intermediate layer in a reducing atmosphere at a temperature up to 750 C., said rhodium layer being suiiiciently thin for diffusion of appreciable and different portions only thereof into said Work piece and said platinum coating, respectively, at said temperature.

11. Method of making a grid comprising shaping a structure of a refractory metal with appreciable force, whereby surface crystals in said structure are ruptured, removing at least some of the oxides in the surface of the shaped structure, electroplating said surface with rhodium, and heating said electroplated surface in a reducing atmosphere, whereby said rhodium coating promotes additional reduction of oxides on said shaped structure for reconstituting said ruptured crystals and increasing the adherence of said coating on said structure.

References Cited in the ile of this patent UNITED STATES PATENTS 

1. A RIGID FOR AN ELECTRON TUBE COMPRISING A REFRACTORY METAL STRUCTURE HAVING A FIRST COATING THEREON OF RHODIUM HAVING A THICKNESS OF FORM 5X10-7 TO 5X10-6 INCH AND A SECOND COATING OF A MATERIAL HAVING A RELATIVELY HIGH WORK FUNCTION ON SAID FIRST COATING.
 10. IN A METHOD OF COATING A MOLYBDENUM ELECTRODE WORK PIECE WITH A RELATIVELY THICK LAYER OF PLATINUM, THE STEPS OF COATING SAID WORK PIECE WITH A RELATIVELY THIN INTERMEDIATE LAYER OF RHODIUM AND HEAT TREATING THE SAID INTERMEDIATE LAYER IN A REDUCING ATMOSPHERE AT A TEMPERATURE UP TO 750*C., SAID RHODIUM LAYER BEING SUFFICIENTLY THIN FOR DIFFUSION OF APPRECIABLE AND DIFFERENT PORTIONS ONLY THEREOF INTO SAID WORK PIECE AND SAID PLATINUM COATING, RESPECTIVELY, AT SAID TEMPERATURE. 